With the recent increase in the amount of data communication, there is a growing need for mobile communication systems having a higher efficiency of frequency usage, and various studies relating to one-cell reuse cellular systems in which all the cells employ the same frequency band are being conducted. In the E-UTRA (Evolved Universal Terrestrial Radio Access) system, which is one of the one-cell reuse cellular systems and under standardization by the 3GPP (3rd Generation Partnership Project), OFDMA (Orthogonal Frequency Division Multiple Access) system is being considered as the most promising candidate of down-link transmission system and SC-FDMA (Single Carrier Frequency Division Multiple Access) system as the most promising candidate of uplink transmission system (see, for example, non-patent document 1).
Although OFDMA is a system by which a terminal accesses in resource block units that are divided by time and frequency using OFDM signals having good tolerance against multi-path fading, it is not suitable for uplink transmission system that severely restricts transmission power due to its high PAPR (Peak-to-Average Power Ratio). On the contrary, SC-FDMA system is suitable for uplink transmission, because it can suppress PAPR to be low in comparison with multi-carrier system such as OFDM so that a wide coverage can be assured.
FIG. 11 shows a configuration of a transmission device (terminal-side transmission device) using SC-FDMA system. In a transmission device using SC-FDMA system, as shown in FIG. 11, the transmission data is first subjected to error correction coding in a coding unit 1000, and modulated in a modulation unit 1001. Next, after being serial-to-parallel converted in an S/P conversion unit 1002, the modulated transmission signal is converted to a frequency domain signal in a DFT (Discrete Fourier Transform) unit 1003.
The transmission signal thus converted to a frequency domain signal as described above is mapped by a subcarrier mapping unit 1004 to a spectrum (subcarrier) to be used for transmission. Here, zero is inserted into a spectrum (subcarrier) that is not used for transmission. Next, the transmission signal mapped by the subcarrier mapping unit 1004 to a spectrum to be used for transmission is input to an IFFT (Inverse Fast Fourier Transform) unit 1005, and converted from a frequency domain signal to a time domain signal. Then, the signal goes through a P/S conversion unit 1006, has a GI inserted therein at a GI (Guard Interval) insertion unit 1007 and, after having been converted to an analog signal by a D/A conversion unit 1008, is up-converted to a radio frequency band signal by a radio unit 1009, and transmitted from an antenna unit 1010.
The transmission signal generated as described above has a lower PAPR than a multi-carrier signal, and has an advantage in that spectrum control can be easily performed because the signal is temporarily converted to a frequency domain signal using DFT. In 3GPP, two types of technique shown in FIG. 12 are proposed as spectrum control methods preserving low PAPR characteristic. The localized allocation shown in FIG. 12A is a technique that preserves a continuous spectrum allocation of transmission signals converted to frequency domain signals by the DFT unit 1003, and the distributed allocation shown in FIG. 12B is a technique that reallocates continuous spectra of transmission signals converted to frequency domain signals by the DFT unit 1003 at a constant interval. When the localized allocation is used, multi-user diversity effect can be obtained by selecting continuous spectra in which signals from respective terminals are received with a high reception power. When the distributed allocation is used, on the other hand, frequency diversity effect can be obtained because spectra are allocated across a wider frequency band than when the localized allocation is used.
In addition, SC-ASA (Single Carrier—Adaptive Spectrum Allocation) system is proposed as a method of realizing a more flexible spectrum control than the two techniques shown in FIG. 12 (for example, non-patent document 2). SC-ASA is a method of freely allocating the spectrum to be used for transmission according to the signal reception condition from each user. The reception characteristic can be largely improved by selecting a spectrum that can obtain high received signal power, although with a slightly higher PAPR than when performing localized allocation or distributed allocation.
With SC-ASA system, PAPR can also be suppressed low by dividing the subcarrier used for transmission into several blocks and continuously mapping subcarriers in a block. It is particularly necessary to reduce PAPR when the possibility that signal distortion may occur rises due to non-linear amplifiers, such as when a terminal is located distant from the base station and high transmission power is required, or when performing transmission in a low power consumption mode with lowered bias voltage to non-linear amplifiers. Therefore, it is important to perform spectrum control (allocation) provided that the subcarriers are grouped into block(s) in varied sizes according to the situation of each terminal. In such a case, terminals with subcarriers grouped into block(s) in varied sizes coexist as simultaneously accessing terminals (which will be referred to as “simultaneous access terminals” as appropriate, hereinafter).
In a cellular system, a plurality of terminals accesses the base station using the transmission system described above to perform data transmission and, according to the non-patent documents 1 and 2, access is performed on a subchannel basis, where subchannels are formed by dividing all the available frequency bands into several segments. Therefore, even a method of flexibly allocating a spectrum to be used, such as SC-ASA system, does not assume to allocate the spectrum beyond the range of a single subchannel, and an adaptive spectrum allocation is performed only within a subchannel.
In a system where a terminal accesses on a subchannel basis as described above, the number of simultaneously accessible terminals is limited to the number of subchannels, and thus it is necessary, before performing spectrum control of each terminal, to select terminals that simultaneously access and allocate a subchannel to each of the selected terminals. Here, round-robin, Max CIR (Carrier-to-Interference power Ratio), or PF (Proportional Fairness) are included in the method of selecting (scheduling) simultaneous access terminals (see, for example, non-patent document 3).
Round-robin is a method of putting all the terminals that have data in a queue and sequentially selecting as many terminals as (at most) the number of subchannels from the queue. A terminal selected from the queue and having finished a certain data transmission is put into the tail of the queue again. Although round-robin can provide equal transmission opportunity to all the terminals, there exists a shortcoming in that cell throughput is limited to be low because channel condition of each terminal is not considered at all. On the other hand, Max CIR selects, as many terminals having the best channel condition as the number of subchannels (at most). When Max CIR is employed, although cell throughput can be maximized, it is very likely that terminals located near the base station have more transmission opportunities, resulting in lack of fairness among terminals.
PF is a method of constantly updating average reception power of each terminal, calculating the difference between the instantaneous reception power and the average reception power, and selecting as many terminals having a large difference as the number of subchannels (at most). Since, according to PF, transmission opportunity is provided to a terminal having the most improved channel condition, cell throughput can be improved compared with the case using round-robin, although PF is not as good as Max CIR. In addition, because the selection is not based on the absolute value of reception power but on the difference between the instantaneous reception power and the average reception power, other terminals as well as those located near the base station are selected, and thereby fairness among terminals can be preserved.
[Non-patent document 1] 3GPP, TR 25.814 v7.1.0, “Physical Layer Aspects for Evolved UTRA”
[Non-patent document 2] IEICE Technical Report RCS2006-233
[Non-patent document 3] 3GPP, TR 25.876 v7.0.0, “Multiple Input Multiple Output in UTRA”